The ability of an animal to withstand and recover from a broad range of environmental challenges, known as resilience, is an emerging field of research in livestock breeding. This thesis explores the potential for using longitudinal wool fibre diameter data to create indicator traits of resilience in sheep. The key research questions addressed were: 1) Can variation in wool fibre diameter be used to assess resilience? 2) What statistical models effectively capture this variation? and 3) How can resilience traits be integrated and valued in sheep breeding programs?
Fibre diameter variation measured along the wool staple was hypothesised to reflect an animal’s capacity to resist, respond and recover from environmental challenges. In the first research chapter (Chapter 3), eight potential resilience indicators were defined from fibre diameter variation along the staple. Heritabilities and genetic correlations were estimated across three age stages: yearling, two-year-old and adult. These traits showed low to moderate heritability (0.01 to 0.33) and were generally well-correlated within age stages. Genetic correlations across age stages were moderate to high between adjacent stages and lower between yearlings and adults. The results suggest genetic improvement is feasible, although refinement is needed to better account for individual and group-level variation along the staple.
To model fibre diameter trajectories, random regression models were applied to longitudinal fibre diameter records in the second research chapter (Chapter 4). Both additive genetic and permanent environmental effects varied along the wool staple, with the ranking of sire breeding value changing with staple position. This suggests that some genotypes were potentially more resilient towards the changes in the growing environment compared to others. The eigenfunctions indicated potential to alter these trajectories and reduce variation.
Resilience indicators were then developed using both longitudinal fibre diameter and body weight data in yearling Merino sheep in Chapter 5. B-splines and a square root velocity function generated mean curves for flock-year groups, enabling correction for temporal variation and calculation of individual deviations. Indicators captured both overall variability and responses to weaning. Correlations with 15 routinely recorded health and production traits were estimated. Although lowly heritable, indicators showed moderate correlations with traits associated with weaning responses, suggesting they reflect meaningful biological variation. Correlations with health and production traits were generally low to moderately favourable, indicating potential for inclusion in breeding programs without compromising productivity.
A multi-trait selection index using a desired gains approach was developed in Chapter 6 to evaluate the impact of including a resilience trait (natural log variance, Lnvar) in a commercial index. Three scenarios were tested: maintaining resilience or improving it by reducing Lnvar by a quarter or half of one genetic SD over 10 years. The required contribution of resilience to the index was estimated at 3.5%, 12.4%, and 25.3%, respectively. Maintaining resilience had minimal impact on productivity, whereas a higher emphasis reduced gains in production traits. This demonstrated that resilience traits can be integrated into selection goals, though detailed modelling of associated costs is warranted.
A deterministic bioeconomic model was developed to estimate the economic value of resilience alongside other production traits in Chapter 7. Benefits of improved resilience included reductions in non-routine health treatments, labour, treatment cost, mortality and nutrient use for fibre production. Resilience accounted for 2% of the total index value, with selection responses producing modest gains (0.08 genetic SD) and an economic return of $0.08 per ewe over 10 years. Sensitivity analyses showed that increasing costs associated with resilience or modifying genetic parameters influenced its index contribution and response. Even at low weightings, resilience contributed to desirable genetic progress without negatively affecting productivity.
The major conclusion of this thesis is that fibre diameter variability along the wool staple offers a promising basis for resilience indicators in sheep. Resilience traits benefit from flexible models that account for the shape of longitudinal data and can be economically valued and integrated into breeding programs.